Radar altimeters of the above type have been described, for example, in "Radio Systems For Location Finding and Navigation" by Kramar, published by: Verlag Berliner Union GmbH, Stuttgart, Federal Republic of Germany. Such systems are, for example, produced and marketed by: AEG (Federal Republic of Germany), Honeywell (U.S.A) and others. The several different versions of such altimeters, however, are subject to a number of disadvantages, especially with regard to their dimensions, their high power consumption, and their high weight due to the analog signal processing. Another disadvantage resides in the fact that particularly the analog signal processing in a continuous operation can be easily discovered, even at long distance where the modulation frequency is controlled in response to the measured altitude. Such signal processing is discovered not only as to its existance, but also the measured altitude can be discovered. Furthermore, many systems of the known type have a rather limited electronic countermeasure resistance and additionally provide faulty altitude measurements when used in forested areas since the analog signal processing even tracks the treetops.